Pistons have been used for years in dispensing cartridges to dispense liquids from the cartridge. The issue has always been, especially with liquids that need to be dispensed at very precise ratios, how to keep air from getting trapped between the piston and the liquid in the cartridge when the piston is inserted into the cartridge during filling. In the very near past, pistons have been developed with integral bleed or ventilation outlets which allow the air to vent to atmosphere without the use of bleed shims or other means to separate the portion of the piston that forms a seal with the cartridge wall from the cartridge wall. These pistons formed with integral bleed or ventilation outlets have been very effective at increasing the efficiency with which air is evacuated from the dispensing cartridge and at decreasing the damage done to piston seals by bleed shims.
While pistons formed with integral bleed or ventilation outlets have been an improvement, significant drawbacks still remain. Such pistons require some separate or specialized device or mechanism to either keep the vent open during the filling process or to close the vent after a cartridge has been filled. None of the prior pistons automatically close during the filling process. None of the prior pistons are self-actuating.
Also, the design of some prior ventilating piston assemblies cause air to be trapped in the piston after the cartridge is sealed. Such trapped air is undesirable. The piston disclosed in U.S. Pat. No. 6,598,766 is an example of such a piston having the undesirable effect of trapping air in the piston after air has been evacuated from the dispensing cylinder.
Accordingly, there is a need for an improved piston for use in a dispensing cartridge.